Hydrogen is universally considered a fuel of the future due to environmental advantages over conventional (i.e., fossil-based) fuels. Another important advantage of using hydrogen stems from the fact that it could be electrochemically (i.e., without Carnot-cycle limitation) converted into electricity with very high energy conversion efficiency using fuel cells (FC).
To be used in energy conversion devices, hydrogen has to be produced and stored; however, each of these aspects of hydrogen technology is associated with major technological challenges.
With regard to production, hydrogen can be produced from hydrocarbon fuels, such as, methane (CH4), and natural gas (NG), via oxidative reforming or thermal (thermocatalytic) decomposition processes.
Oxidative reforming involves the reaction of hydrocarbons with oxidants: water, oxygen, or a combination thereof; the corresponding processes are steam reforming, partial oxidation and autothermal reforming, respectively. As a first step, these processes produce a mixture of hydrogen with carbon monoxide (synthesis-gas), which is followed by water gas shift and CO2 removal stages. The total CO2 emissions from these processes exceed 0.4 m3 per each m3 of hydrogen produced.
Thermal (thermocatalytic) decomposition or dissociation of hydrocarbons occurs at elevated temperatures (500-1500° C.) in an inert (or oxidant-free) environment and results in the production of hydrogen and elemental carbon. Due to the lack of oxidants, no carbon oxides are produced in the process. This eliminates or greatly reduces carbon dioxide (CO2) emissions and obviates the need for water gas shift and CO2 removal stages, which significantly simplifies the process. The process produces pure carbon as a valuable byproduct that can be marketed, thus reducing the net cost of hydrogen production. The following is a brief description of the prior art with regard to hydrocarbon thermal (thermocatalytic) decomposition technologies.
Thermal decomposition of natural gas (NG), known as the Thermal Black process, has been practiced for decades as a means of production of carbon black (Kirk-Othmer Encyclopedia of Chemical Technology, vol. 4, pages 651-652, Wiley & Sons, 1992). In this process a hydrocarbon stream was pyrolyzed at high temperature (1400° C.) over the preheated contact (firebrick) into carbon black particles and hydrogen, which was utilized as a fuel for the process. The process was employed in a semi-continuous (i.e., cyclic pyrolysis-regeneration) mode using two tandem reactors.
U.S. Pat. No. 2,926,073 to Robinson et al. describes the improved continuous process for making carbon black and byproduct hydrogen by thermal decomposition of natural gas (NG). In this process, NG is thermally decomposed to carbon black and hydrogen gas is used as a process fuel in a bank of heated tubes at 982° C.
Thus, both technological approaches described above, target the production of only one product: carbon black, with hydrogen being a supplementary fuel for the process.
Kvaerner Company of Norway has developed a methane decomposition process, which produces hydrogen and carbon black by using high temperature plasma (CB&H process described in the Proceedings of 12th World Hydrogen Energy Conference, Buenos Aires, p. 637-645, 1998). The advantages of the plasmochemical process are high thermal efficiency (>90%) and simplicity, however, it is an energy intensive process.
Steinberg et al. proposed a methane decomposition reactor consisting of a molten metal bath in Int. J. Hydrogen Energy, 24, 771-777, 1999. Methane bubbles through molten tin or copper bath at high temperatures (900° C. and higher). The advantages of this system are: an efficient heat transfer to a methane gas stream and ease of carbon separation from the liquid metal surface by density difference.
Much research on methane decomposition over metal and carbon-based catalysts has been reported in the literature. Transition metals (e.g. Ni, Fe, Co, Pd, and the like.) were found to be very active in methane decomposition reaction; however, there was a catalyst deactivation problem due to carbon build up on the catalyst surface. In most cases, surface carbon deposits were combusted by air (or gasified by steam) to regenerate the catalyst's original activity resulting in large amounts of CO2 byproduct.
For example, Callahan describes “a fuel conditioner” designed to catalytically convert methane and other hydrocarbons to hydrogen for fuel cell applications in Proc. 26th Power Sources Symp. Red Bank, N.J., 181-184, 1974. A stream of gaseous fuel entered one of two reactor beds, where hydrocarbon decomposition to hydrogen took place at 870-980° C. and carbon was deposited on the Ni-catalyst. Simultaneously, air entered the second reactor where the catalyst regeneration occurred by burning coke off the catalyst surface. The streams of fuel and air were reversed for another cycle of decomposition-regeneration. The reported process did not require water gas shift and gas separation stages, which was a significant advantage. However, due to cyclic nature of the process, hydrogen was contaminated with carbon oxides. Furthermore, no carbon byproduct was produced in this process.
U.S. Pat. No. 3,284,161 to Pohlenz et al. describes a process for continuous production of hydrogen by catalytic decomposition of NG. Methane decomposition was carried out in a fluidized bed catalytic reactor in the range of temperatures from 815° C. to 1093° C. Supported Ni, Fe and Co catalysts (preferably, Ni/Al2O3) were used in the process. The deactivated (coked) catalyst was continuously removed from the reactor to the regenerator where carbon was burned off, and the regenerated catalyst was recycled to the reactor.
U.S. Pat. No. 2,476,729 to Helmers et al. describes the improved method for catalytic cracking of hydrocarbon oils. It was suggested that air is added to the feedstock to partially combust the feed such that the heat supplied is uniformly distributed throughout the catalyst bed. This, however, would contaminate and dilute hydrogen with carbon oxides and nitrogen.
Use of carbon catalysts offers the following advantages over metal catalysts: (i) no need for the regeneration of catalysts by burning carbon off the catalyst surface, (ii) no contamination of hydrogen by carbon oxides, and (iii) carbon is produced as a valuable byproduct of the process. Muradov has reported on the feasibility of using different carbon catalysts for methane decomposition reaction in Energy & Fuel, 12, 41-48, 1998; Catalysis Communications 2, 89-94, 2001.
U.S. Pat. No. 2,805,177 to Krebs describes a process for producing hydrogen and product coke via contacting a heavy hydrocarbon oil admixed with a gaseous hydrocarbon with fluidized coke particles in a reaction zone at 927° C.-1371° C. Gaseous products containing at least 70 volume % of hydrogen were separated from the coke, and a portion of coke particles was burnt to supply heat for the process; the remaining portion of coke was withdrawn as a product.
U.S. Pat. No. 4,056,602 to Matovich teaches high temperature thermal decomposition of hydrocarbons in the presence of carbon particles by utilizing fluid wall reactors. Thermal decomposition of methane was conducted at 1260° C.-1871° C. utilizing carbon black particles as adsorbents of high flux radiation energy, and initiators of the pyrolytic dissociation of methane. It was reported that 100% conversion of methane could be achieved at 1815° C. at a wide range of flow rates (28.3-141.5 l/min).
U.S. Pat. No. 5,650,132 to Murata et al. describes the production of hydrogen from methane and other hydrocarbons by contacting them with fine particles of carbonaceous materials. The carbonaceous materials included carbon nanotubes, activated charcoal, fullerenes C60-C70, finely divided diamond powder as well as soot obtained by thermal decomposition (or combustion) of different organic compounds or by arc discharge between carbon electrodes in vacuum. The optimal conditions for methane conversion included: preferable methane concentration: 0.8-5 volume % (balance inert gas), the temperature range of 400° C.-1,200° C. and residence times 0.1-50 sec.
U.S. Pat. No. 6,670,058 to Muradov describes the continuous process for hydrogen and carbon production using carbon-based catalysts. The process employs two fluid-solid vessels: a reactor and a heater/regenerator with carbon particles circulating between the vessels in a fluidized state. NG enters a fluidized bed reactor (FBR) where it is decomposed over a fluidized bed of catalytically active carbon particulates at the temperature range of 850° C.-900° C. The resulting hydrogen-rich gas enters a gas separation unit where a stream of hydrogen with a purity of >99.99 volume % is separated from the unconverted methane, which is recycled to the FBR. The carbon particles are directed to a fluidized bed heater where they are heated to 1000-1100° C. by hot combustion gases containing steam and CO2, and simultaneously activated. The main portion of carbon particles is withdrawn from the system as a product.
In summary, the major problem with respect to metal- and carbon-catalyzed decomposition of hydrocarbons relates to gradual deactivation of the catalysts during the process. The deactivation could mainly be attributed to the inhibition of the catalytic process by the carbon deposits blocking the catalyst active sites. This necessitates the regeneration of the catalysts either by complete combustion or gasification of the carbon deposits, in case of metal catalysts or partial gasification of carbon deposits, in case of carbon-based catalysts.
The regeneration step significantly complicates the process and results in contamination of hydrogen with carbon oxides, necessitating an elaborate hydrogen purification step and production of considerable amount of CO2 emission. Thus, there is a need for a more efficient, simple, versatile and sustainable process for the production of hydrogen and carbon from different hydrocarbons without catalyst regeneration and with drastically reduced CO2 emission when compared to conventional processes.
The present invention improves upon and overcomes many of the deficiencies of the prior art.